Displacement transducer

ABSTRACT

A displacement transducer includes a load cell structure having a thick outer peripheral area, a thick inner central area and two symmetrical thin beams. The two beams are disposed along a common diameter of the structure and joins the outer peripheral area and the inner central area. At least one strain gauge is placed on a surface of one beam and at least one strain gauge is placed on a surface of the second beam. A top diaphragm cover member is secured to a top surface of the outer peripheral area and covers the two beams.

CLAIM FOR PRIORITY

This application is a continuation of application Ser. No. 11/322,721filed Dec. 30, 2005, the entire disclosure of which is herebyincorporated by reference as if being set forth in its entirety herein

FIELD OF THE INVENTION

The present invention relates generally to displacement sensors and moreparticularly to a hermetically sealed displacement sensor usingpiezoresistors.

BACKGROUND OF THE INVENTION

Semiconductor piezoresistive transducers and displacement sensors havebeen widely known and are used in a great variety of applicationsincluding those applications having extremely harsh environments. Suchdevices may employ semiconductor or metal strain gages depending on theenvironment and application. For such applications, sensors have to beprotected from the environment. For many applications the sensors mustbe contained within a cavity (usually a vacuum) to protect the straingages. This vacuum cavity is hermetically sealed to maintain the vacuumand protect the sensing elements. Such protected sensors may be employedin displacement pressure sensors and used in many applicationsincluding, for example, medical, automotive, and aerospace applications.

Techniques for hermetically sealing semiconductor piezoresistors fromhostile environments have generally limited the size of such devices.The reason for this is that additional lateral space is required toaccommodate the hermetically sealing cover member. The piezoresistivetransducer employs silicon resistive elements whose resistance variesaccording to intensity or magnitude of applied displacement upon anassociated diaphragm. These resistors must be hermetically isolated fromthe external environment to ensure proper sensing performance and avoiddestruction in harsh conditions.

For an example of hermetically sealed environmentally protectedtransducers, reference is made to U.S. Pat. No. 5,002,901 entitled“Method of Making Integral Transducer Structures Employing HighConductivity Surface Features”, issued on Mar. 26, 1991 to A. D. Kurtz,et al and assigned to the assignee herein. In this patent thepiezoresistive elements are formed over the central region of adielectric layer which overlays a silicon diaphragm. The elements arearranged to form a Wheatstone bridge where four circuit nodes of thebridge are configured as four p+ silicon electrical contact postsdisposed on the peripheral corners of the device. Electricalinterconnections also comprised of p+ silicon interconnect the contactposts with the piezoresistive transducer elements. A bias voltage isbrought to the two contacts where the voltage is measured between theother two contacts. In this manner the hermetic seal for this device isprovided by fabricating the peripheral flange on the device's outerperiphery beyond the contact posts, and an absolute cavity can be madewhich provides a vacuum reference. A glass sheet cover is then bonded tothe top of the flange to create the hermetic seal. U.S. Pat. No.5,891,751 entitled “Hermetically Sealed Transducers and Methods forProducing the Same”, issued on Apr. 6, 1999 to A. D. Kurtz, et al andassigned to Kulite Semiconductor Products, Inc., the assignee herein,teaches a hermetically sealed semiconductor transducer and methods forfabricating the same. In this patent a sealing member hermetically sealsan aperture whereby a vacuum is maintained between the transducerelement and the cover member. The transducer element is hermeticallysealed from the external environment while at least a portion of theelectrical contact remains exposed to enable subsequent wire bondingthereto.

Reference is also made to U.S. Pat. No. 5,461,001 entitled “Methods forMaking Semiconductor Structures Having Environmentally IsolatedElements”, issued on Oct. 24, 1995 to A. D. Kurtz, et al and assigned tothe assignee herein. This patent shows a method of fabricatingsemiconductor structures where one can provide a great number ofhermetically sealed individual circuit devices using the methodsdescribed in the above noted patent. Reference is also made to U.S. Pat.No. 6,229,427 entitled “Covered Sealed Pressure Transducers and Methodsfor Making the Same”, issued on May 8, 2001 to A. D. Kurtz, et al andassigned to the assignee herein. That patent shows a method which can beutilized to hermetically seal a raised feature of a sensing network of asilicon on oxide pressure transducer. The invention described in the'427 patent can also be utilized to hermetically seal the depressedfeature sensing network of a diffused pressure transducer.

U.S. Pat. No. 5,286,671 entitled “Fusion Bonding Technique for Use inFabricating Semi Conductor Devices”, issued to A. D. Kurtz, et al onFebruary 1994, relates to silicon oxide pressure transducers and methodsfor fabricating and bonding to such devices.

In view of the above it is extremely desirable to produce a hermeticallysealed displacement sensor, which is easy to fabricate and whichprovides improved operation over prior art devices.

SUMMARY OF THE INVENTION

A hermetically sealed displacement transducer, comprising a load cellstructure having a top surface and a bottom surface and having a thickouter peripheral area and a thick inner concentric central area withsaid outer peripheral area and said inner central area joined by twosymmetrical thin beams directed along a common diameter with a firstbeam directed from an inner edge of said peripheral area to an outeredge of said central area, with a second beam directed along saiddiameter from an oppositely opposed inner edge of said peripheral areato an oppositely opposed outer edge of said inner area, said beams belowthe top surfaces of said peripheral and central areas and above thebottom surfaces thereof, at least a first strain gauge positioned on asurface of one beam, and at least a second strain gauge positioned on acorresponding surface of said second beam, a bottom cover member securedto said bottom surface of said peripheral area to cover and enclose saidbeams and said strain gauges, a top diaphragm cover member secured tosaid top surface of said peripheral area to cover and enclose saidbeams, with said cover member and said diaphragm member forming ahermetic cavity for said beams and strain gauges.

BRIEF DESCRIPTION OF THE FIGURES

Understanding of the present invention will be facilitated byconsideration of the following detailed description of the preferredembodiments of the present invention taken in conjunction with theaccompanying drawings, in which like numerals refer to like parts and inwhich:

FIG. 1 is a top view of a displacement sensor beam according to thisinvention;

FIG. 2 is a cross sectional view taken through line 2-2 of the load cellshown in FIG. 1. FIG. 2 includes an isolation diaphragm and a bottomcover member;

FIG. 3 is a top plan view of a convoluted isolation diaphragm shown inthis invention;

FIG. 4 is a sectional view showing the shape of a particularconvolution;

FIG. 5 is a detailed view depicting the area shown in FIG. 2 within adashed circle; and,

FIG. 6 is an enlarged sectional view depicting the area shown within thedashed circle of FIG. 2 designated as FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

It is to be understood that the figures and descriptions of the presentinvention have been simplified to illustrate elements that are relevantfor a clear understanding of the present invention, while eliminating,for purposes of clarity, many other elements found in typicaldisplacement sensor systems and methods. However, because such elementsare well known in the art, and because they do not facilitate a betterunderstanding of the present invention, a discussion of such elements isnot provided herein. The disclosure herein is directed to all suchvariations and modifications known to those skilled in the art.

Referring to FIG. 1 there is shown a top view of a load cell beamarrangement 10. According to an embodiment of the present invention, theload cell and beams are integrally formed from a suitable metal. FIG. 2depicts a cross sectional view of the load beam cell configuration ofFIG. 1 taken through line 2-2 of FIG. 1. The load beam cell of FIG. 1 isshown in a top view and essentially is circular in configuration, butother geometrical configurations can be employed as well, including butnot limited to rectangular, oval, square, and/or other geometries. Asone can ascertain from FIG. 1, the load cell 10 has a thickened outerperipheral area 35 which is depicted in FIG. 2 as well. The outer area35 is integral with two beams 11 and 12. While two beams 11 and 12 areshown along the common diameter 26, it is understood that additionalbeam pairs may be disposed along common diameters of the load cell. Forexample, two beams may be placed along a diameter which is transverse todiameter 26. These beams do not necessarily require strain gages, butare formed with the load cell to prevent twisting and/or sideloading.The beams 11 and 12 as shown in FIG. 2 are thinned areas capable ofdeflecting. Both beams 11 and 12 are generally triangular in shape andextend from a central hub 19, which central hub 19 is common to bothbeams 11 and 12 and each beam 11, 12 is directed along a common diameter26. The beams 11 and 12 as seen extend from the central hub 19 along acommon diameter 26 and gradually increase in size as they meet with thecircular edge 40 of the load beam cell 10. The beams 11 and 12 aregenerally triangular having the base of the triangle at the edge 40 ofthe inner surface of area 35, and with the truncated apex integral withthe outer periphery of hub 19. Thus, the corresponding beams 11 and 12with the common central hub 19 are of a bow-tie configuration. However,other configurations may be used as well.

Each beam 11, 12 has located thereon piezoresistive sensors as 14 and 15associated with beam 11 and 16 and 17 associated with beam 12. Each ofthe piezoresistive sensors 14-17 is arranged so that one is positionedin a longitudinal direction and the other is in a transverse directionwith respect to diameter 26. Piezoresistive sensors 14-17 are silicondevices and are well known as being fabricated by many differenttechniques. In any event, each of the piezoresistive silicon devices14-17 has leads such as lead 23 associated with device 16. The leads 23are directed through a common channel 20 which is directed from theperiphery of the inner circle 40 to the outside of the load cell 10.This channel accommodates wires from each of the piezoresistive sensors.Typically the piezoresistive sensors 14-17 are connected to form a fullbridge such as a Wheatstone bridge whereby the output of the bridgewould be proportional to a stress supplied to the beams 11 and 12. Asseen in FIG. 2, each beam such as 11 and 12 is thin compared to thethicker outer peripheral member 35. The beams 11 and 12 are integrallyformed therewith and are much thinner than the outer peripheral area 35.In a typical example, the outer peripheral area 35 may be about 0.15inches thick with the beams being about 0.04 inches thick. The diameterof the cell 10 shown in FIG. 1, which is the outer diameter, istypically about three (3) inches. The beams 11 and 12 as provided aretwo constant moment beams joined at the center via this central hub 19.As seen the central hub 19 has an outer area 18 which is depressed orlocated below the inner area 19. This depression is approximately 0.002inches in height and is a recess to accommodate a top convoluteddiaphragm cover. Peripheral region 35 also has apertures such as 25located about the periphery, which apertures serve as mounting aperturesto enable the entire load cell 10 to be mounted on a surface. Therefore,a pressure can be applied to the cell 10 and which pressure would bedirected to the thin beam sections 11 and 12 of the load cell 10(FIG.1).

Referring to the cross sectional view of FIG. 2, it is seen that thepiezoresistor sensors 14, 15, 16 and 17 are enclosed within ahermetically sealed cavity 38. The cavity 38 is formed by placing acover member 30 over the bottom surface of the cell 10 shown in FIG. 1.The cover member 30 is attached (e.g. welded) to the outer concentricperipheral area 35 of the cell. As seen in FIG. 2, there is a stepdepression 37 whereby the outer periphery of the cover member 30 isthinner and fits within the peripheral depression 37 formed in the outerthick concentric portion 35. A top cover 31 contains a series ofconvolutions (42 a, 42 b, . . . 42 k) and essentially acts as anisolation diaphragm for the sensing device. The convoluted isolationdiaphragm deflects for forces applied thereto are transmitted to thetriangular beams 11 and 19 and are responded to by the hermeticallysealed sensors. The cover member 30 is spaced apart from the central hub19 by a space 36 to enable the diaphragm to deflect upon application ofa force thereto. The force is applied essentially at the center wherebythe triangular shaped thin beams act as two constant moment beams whichare joined at the center portion.

FIG. 3 depicts the top isolation diaphragm 31 showing the particularshape of the convolutions in FIG. 4. As seen in FIG. 4, the convolutedisolation diaphragm 41 has a central aperture 40 which central aperturesurrounds the raised impression 19 (shown in FIG. 1). The rim orconcentric area 43 sits on top of the depressed area 18 allowing theisolation diaphragm 31 to be positioned as shown with a predeterminedspace 36 from the thin triangular beam sections 11 and 12. Theconvolution 42 is shown in detail in FIG. 4 and is round with the topfacing the beams 11 and 12.

FIGS. 5 and 6 show the encircled sections of FIG. 2 in greater detail.From FIG. 5 and FIG. 6, one can visualize the relationship of theconvoluted isolation diaphragm 31 and the bottom sealing cover 30including the spacing 36 between the bottom cover member 30 and thecentral hub portion 19 of the cell. FIG. 6 again depicts the encircledarea of FIG. 2 designated by the dashed circle labeled FIG. 6 and alsoshows the beam 12 which is integrally formed with the thick outerconcentric peripheral area 35 showing the cover member 30 which iswelded to the area 35 and showing the convoluted isolation diaphragm 31in greater detail. The dimensions of course can vary for variousstructures but consistent with the above noted dimensions and referringto FIG. 5 it is indicated that the radius 43 depicted is typically 0.01inches with the thickness of the convoluted cover member 31 being 0.002inches and with the spacing 36 from the bottom of the member 19 to theoutside of the cover 30 being 0.04 inches. In FIG. 6, the thickness ofthe bottom cover 30, which hermetically seals the sensors, is 0.015inches. As previously indicated the thickness of the outer concentricarea 35 is 0.14 inches while the thickness of the beams as 11 and 12 areabout 0.02 inches. Thus, the device 10 has strain gauges orpiezoresistors 14 to 17 placed on the flexible thin beams 11 and 12. Thegauges are in a hermetically sealed cavity 38 which is formed by a topcover diaphragm member 31 and a bottom cover 30. The sensors 14-17 areplaced on the beams 11 and 12 by use of an apoxy or bonding agent. Thebeams 11 and 12 are two constant moment beams joined at the center. Thethin beams 11 and 12 provide a compliant member to allow displacement ofthe sensor. The displacement produces a strain in the thin beams 11 and12 and by measuring the strain, one obtains an electrical outputproportional to the displacements. The electrical output from the deviceis proportional to the deflection of the center of the sensor. Whiletypical dimensions were given above for a typical sensor it isunderstood that dimensions of the beams as well as thickness anddiameters are selected to give the required displacement with minimalforce imparted on the measured device.

One skilled in the art will understand how to formulate such dimensionsdepending upon the application. It is therefore apparent that there aremany modifications which can be imparted by one skilled in the art. Allsuch modifications are deemed to be encompassed in the spirit and scopeof the enclosed claims.

It will be further apparent to those skilled in the art thatmodifications and variations may be made in the apparatus and process ofthe present invention without departing from the spirit or scope of theinvention. It is intended that the present invention cover themodification and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

1. A displacement transducer, comprising: a load cell structure having athick outer peripheral area, a thick inner central area, and first andsecond symmetrical thin beams disposed along a first common diameter ofsaid cell structure and joining said outer peripheral area and saidinner central area; at least a first strain gauge positioned on asurface of said first beam, and at least a second strain gaugepositioned on a corresponding surface of said second beam; and a topdiaphragm cover member secured to a top surface of said outer peripheralarea to cover said first and second beams.
 2. The displacementtransducer according to claim 1, wherein said outer peripheral area,said inner central area and said first and second beams are integrallyformed.
 3. The displacement transducer according to claim 1, whereinsaid outer peripheral area includes a bottom surface opposite said topsurface, said bottom surface having a peripheral depression along aportion thereof, said peripheral depression adapted to receive a bottomcover member.
 4. The displacement transducer according to claim 3,further comprising a bottom cover member secured to said bottom surfaceof said peripheral area to cover and enclose said first and second beamsand said strain gauges, wherein said cover member and said diaphragmmembers form a hermetically sealed cavity for said first and secondbeams and said strain gauges, wherein a gap is formed between saidbottom cover and said inner central area.
 5. The displacement transduceraccording to claim 1, wherein said at least a first strain gaugecomprises a first longitudinal strain gauge mounted on said first beamand a second transverse strain gauge also mounted on said first beam,and wherein said at least a second strain gauge comprises a thirdlongitudinal strain gauge mounted on said second beam in a locationcorresponding to the location of said first longitudinal gauge on saidfirst beam and a fourth transverse strain gauge mounted on said secondbeam in a location corresponding to said second transverse beam locationof said second gauge.
 6. The displacement transducer according to claim1, further comprising a third and a fourth beam disposed along a secondcommon diameter between said outer peripheral area and said innercentral, wherein said third beam is directed from an inner edge of saidperipheral area to an outer edge of said inner central area, and saidfourth beam is directed along said diameter from an oppositely opposedinner edge of said peripheral area to an oppositely opposed outer edgeof said inner central area.
 7. The displacement transducer according toclaim 1, wherein said first and second beams are two constant momentbeams joined at their centers.
 8. The displacement transducer accordingto claim 1, wherein each of said first and second beams is generally ofa triangular configuration with an apex end of said configurationlocated at an outer edge of said inner central area and with the base ofsaid configuration located at an inner edge of said outer peripheralarea.
 9. The displacement transducer according to claim 1, wherein saidgauges are silicon piezoresistors.